Transformations of taxol

ABSTRACT

A method for esterifying C13 deoxy taxoid intermediates employs three steps, i.e., oxygenation of the C13 deoxy taxoid intermediate to produce a C13 enone taxoid intermediate; reduction of the C13 enone to produce an alcohol; followed by esterification of the C13 alcohol. Key intermediates include C13 deoxy taxoids; C13 enone substituted taxoids; and C1-C2 cyclo carbonate esters of taxoids.

The instant case is a continuation-in-part from application Ser. No.08/110,095 filed Aug. 20, 1993, now U.S. Pat. No. 5,440,057.

FIELD OF INVENTION

The invention relates to taxol and to the synthesis of taxol analogs.More particularly, the invention relates to processes and keyintermediates for synthesizing taxol analogs.

BACKGROUND

Taxol is a natural product with anti-cancer activity. Because naturalsources of taxol are limited, synthetic methods for producing taxol havebeen developed, e.g., K. C. Nicolaou et al., J. Chem. Soc., Chem.Commun. 1992, 1117-1118, J. Chem. Soc., Chem. Commun. 1992, 1118-1120,and J. Chem. Soc., Chem. Commun. 1993, 1024-1026. Several synthetictaxol analogs have also been developed and have been found to havealtered chemical and biological activity as compared to natural taxol,e.g., K. C. Nicolaou et al., Nature, 1993, 364, 464-466. There isconsiderable interest in the design and production of further taxolanalogs. However, progress with respect to the synthesis of such taxolanalogs has been blocked by a lack of information regarding certain keysynthetic methods and key intermediates essential for the production ofa wide range of taxol analogs.

What is needed is the identification of key synthetic methods and keyintermediates for producing taxol analogs having altered activities.

SUMMARY

One aspect of the invention is directed to a method for esterifyingtaxoid intermediates having an ABCD ring skeleton structure with ringcarbons C1-C15 and C20 represented by the following structure: ##STR1##wherein the C13 carbon is a deoxy carbon. The method employs includes atleast three steps. In the first step, the deoxy C13 of the taxoidmolecule is oxygenated to form a C13 ketone. Pyridinium chlorochromateis a preferred oxidant for performing this process. In the second step,the C13 ketone produced in the first step is reduced to form a C13alcohol. Sodium borohydrate is a preferred reductant to form an alcoholfrom the C13 ketone. In the third step, C13 alcohol formed in the secondstep is esterified. A preferred method of esterification employs aβ-lactam intermediate as taught by Ojima (Ojima, I. et al., Tetrahedron1992, 48, 6985 and Tetrahedron Lett. 1993, 34, 4149) and by Holton, R.(European Patent Application No. EP 400,971 (1990) and Chem Abstracts1990, 114, 164568q).

An alternative aspect of this invention is directed to an improvedtaxoid intermediate having an ABCD ring skeleton structure with ringcarbons C1-C15 and C20 as indicated above wherein the C1 and C2 carbonsare incorporated within a cyclo carbonate ester. An example of such animproved taxoid intermediate is indicated below: ##STR2## wherein R isselected from the group consisting of H and a protective group forhydroxyls.

A further aspect of the invention is directed to an improved taxoidintermediate having an ABCD ring skeleton structure with ring carbonsC1-C15 and C20 the C13 carbon is a deoxy carbon. Examples of a C13 deoxytaxoid intermediate is provided below: ##STR3## wherein R is selectedfrom the group consisting of H and a protective group for hydroxyls.

Another alternative aspect of this invention is directed an improvedtaxoid intermediate having an ABCD ring skeleton structure wherein theC13 carbon includes a ketone substitution and forms an enone with theC12-C11 bridgehead double bond. An example of this aspect of theinvention is illustrated by the following structure: ##STR4## wherein Ris a protective group for hydroxyls, preferably SiEt₃.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates degradative and synthetic plans for producing taxoidintermediates from naturally occurring 10-deacetyl baccatin III (2) andfor converting such taxoid intermediates to taxol (1).

DETAILED DESCRIPTION

Chemistry is disclosed which defines chemical pathways via which taxol 1and 10-deacetyl baccatin III 2 (Indena Company, Italy) can be convertedto a variety of intermediates including compounds 4-6 and 12-15, all ofwhich can then be converted back to taxol 1. These reactions can beemployed in the preparation of taxol analogs and in the total synthesisof taxol.

Initially, a C1-C2 vicinal diol was prepared in order to study theintroduction of protecting groups at the C2 position and theirconversion to the C1 hydroxy-C2 benzoate. To this end, 7-SiEt₃ baccatinIII (3) was prepared from 10-deacetyl baccatin III (2) according to themethods of Magri et al (Journal of Organic Chemistry 1986, 51, p. 3239)and of Denis et al. (Journal of the American Chemical Society 1988, 110,p. 5917), as shown in FIG. 1. All attempts to selectively deprotect theC2 and C10 hydroxyl groups, including basic hydrolysis and metal hydridereductions, produced only low yields of the desired triol. It was thenpostulated that oxidation of the C13 hydroxyl group would remove apossible hydrogen bond between that hydroxyl and the C4 acetate, thusrendering the C4 acetate less susceptible to hydrolysis orintramolecular attack from the C2 hydroxyl group. Indeed, TPAP oxidationof compound 3, according to the method of Griffith (Aldrichim. Acta,1990, 23, 13), provided the corresponding C13 ketone, in 98% yield. Thiswas readily hydrolyzed under basic conditions to provide thecorresponding C1-C2-vicinal diol with an 81% yield.

Modeling studies (Nicolaou et al., J. Chemical Society, Chem.Communications, 1992, p. 1118) suggest the benefit of using a cyclicprotecting group for the C1-C2 diol in order to preorganize themolecular skeleton prior to ring closure to form the 8-membered ring.Furthermore, with the goal of selectively introducing the C2 benzoylgroup in the synthetic direction, we found that it is possible todirectly convert a C1-C2 carbonate ester into a C2 benzoate by additionof a nucleophilic reagent carrying a phenyl group. Treatment of thetriol resulting from the oxidation/hydrolysis of 3 with a phosgene inpyridine, did indeed provide the desired carbonate 4 with a yield of65%. The acetate 5 was then prepared from 4 using standard acetylationconditions. This intermediate (5) served admirably as a precursor oftaxol (1) as described below.

Treatment of carbonate 5 with excess PhLi at -78° C. for 10 minutesresulted in the regioselective formation of the benzoate 6 with a yield,according to chromatographic and spectroscopic analysis, of 70%. A smallamount (approximately 1-%) of the 10-deacetyl product resulting fromPhLi attack on the C10 acetate group was also observed, althoughtreatment of the crude reaction mixture with Ac₂ O in the presence ofDMAP provided 6 as a single product, raising the yield of the 5 to 6step to 80%. This chemistry provided a convenient protecting device forthe C1-C2 diol group and opened directed access to the C1 hydroxyl/C2benzoate system of taxol. The use of other nucleophilic reagentscarrying other than phenyl groups to selectively open this carbonatering should provide a variety of C2 ester, a class of derivatives whichis otherwise difficult to obtain from naturally occurring taxoids. Theremarkable resistance of the other four carbonyl functionalities incompound 5 towards PhLi is presumably due to steric shielding of thesesites. Conversion of enone 6 back to taxol (1) was then demonstrated bythe following sequence. Regio- and stereoselective reduction of the C13carbonyl group was achieved with NaBh₄, resulting in the formation of7-TES (SiEt₃) baccatin III (3) in 83% yield, according to the method ofKingston, (Pharmac. Ther. 1991, 52, p. 1). Attachment of the side chainonto intermediate 3 was then accomplished using Ojima's method, i.e.,Ojima, et al., Tetrahedron 1992, 48, 6985 and Tetrahedron Lett. 1993,34, 4149 and Holton, R., European Patent Application No. EP 400,971,filed 1990 and Chem Abstracts 1990, 114, 164568q. Thus optically activeβ-lactams 7 and 8 were coupled with 3 using NaN(SiMe₃)₂, to provide2',7-diprotected taxol intermediates 9 and 10 respectively. Deprotectionof either of these compounds (9 or 10) using standard conditionsprovided taxol 1 with a overall yield from 3 of approximately 70%.

Anther possible step in a potential total synthesis of taxol 1 is theoxidation of the C13 methylene to a ketone group. To test thishypothesis, the C13 deoxy compound 12 was prepared from 3, via thethionoimidozolide 11, using Barton's deoxygenation procedure, i.e.,thiocarbonyldiimidazole-DMAP, heat, 86%, followed by Bu^(n) ₃ SnH--AIBN,heat, 40%. (Barton, J. Chem. Soc., Perkin I 1975, p. 1574 and Hartwig,Tetrahedron 1983, 39, p. 2609.) A substantial amount, approximately 25%,of the corresponding C12-C13 alkene was also isolated in thisdeoxygenation reaction. Enone 6 was then prepared from 12, with a yieldof 75%, using pyridinium chlorochromate (PCC) in refluxing benzene. Inorder to penetrate further into the projected synthetic scheme, the7-hydroxy compound 13 was prepared from 12 by desilylation (HF.pyr,65%). Conversion of compound 13 back to 12 was accomplished using Et₃SiCl in pyridine, with a yield of 85%.

Compound 12 was also converted to carbonate 14 using similar chemistryas described for the synthesis of 4, i.e., K₂ CO₃ in MeOH/H₂ O/THF, 85%based on 55% conversion followed by phosgene in pyridine, 95%.Desilylation of 14 (HF.pyr, 88%) led to the 7-hydroxy compound 15 whichwas converted back to 14 by silylation under standard conditions, i.e.,Et₃ SiCl--pyr, 85%. Nucleophilic addition of PhLi to the carbonate 14 asdescribed above provided the benzoate 12 with a yield of 80%.

REAGENTS AND CONDITIONS

The reagents and conditions for the reactions indicated in FIG. 1 areprovided below:

(i) To 10-deacetyl baccatin III 2 is added 20 equivalents of Et₃ SiCl inpyridine at 25° C. for 20 hours to produce the TES (SiEt₃) intermediatewith a yield of 89%.

(ii) To the TES product of (i) is added 5 equivalents of AcCl inpyridine at 0° C. for 48 hours to produce 7-TES baccatin III 3 with ayield of 90%.

(iii) To the product of (iii) is added 0.05 equivalents of (Pr^(n))₄NRuO₄, 1.5 equivalents of 4-morpholine N-oxide, 4 Å molecular sieves inacetonitrile for 30 minutes with a yield of 98%.

(iv) To the product of (iii) is added K₂ CO₃ cat., in MeOH, H₂ O at 0°C. for 9 hours with a yield of 81%.

(v) To the product of (iv) is added 10 equivalents of phosgene inpyridine at 25° C. for 30 minutes to produce compound 4 with a yield of65%.

(vi) To compound 4 is added 10 equivalents of Ac₂ O and 20 equivalentsof 4-dimethylaminopyridine in Ch₂ Cl₂ for 30 minutes to produce compound5 with a yield of 95%.

(vii) To compound 5 is added 5 equivalents of PhLi in TNF at -78° C. for10 minutes to produce compound 6 with a yield of 70% plus 10%10-deacetyl 6.

(viii) To compound 6 is added 10 equivalents of NaBH₄ in MeOH at 25° C.for 5 hours to produce compound 3 with a yield of 83%.

(ix) To compound 3 is added 3.5 equivalents of 7 or 8 and 3 equivalentsof NaN(SiMe₃)₂ in THF at 0° C. for 30 minutes to produce compounds 9 or10 respectively with a yield of 87% based upon 90% conversion.

(x) To compound 9 is added HF.pyridine in THF at 25° C. for 1.25 hoursto produce compound 1 with a yield of 80%. To compound 10 is added EtOh,0.5% HCl at 0° C. for 72 hours to produce compound 1 with a yield of80%.

(xi) To compound 3 is added 20 equivalents of thiocarbonyldiimidazoleand 30 equivalents of 4-dimethylaminopyridine in THF in sealed tubes at75° C. for 18 hours to produce compound 11 with a yield of 86%.

(xii) To compound 11 is added 20 equivalents of Bu^(n) ₃ SnH, AIBN cat.,in toluene at 65° C. to produce compound 12 with a yield of 40%, plus25% of C12-C13 alkene.

(xiii) To compound 12 is added 30 equivalents ofpyridiniumchlorochromate, NaOAc, Cellite in refluxing benzene to producecompound 6 with a yield of 75%.

(xiv) To compound 12 or 14 is added HF.pyridine in THF at 25° C. for 1hour to produce compound 13 with a yield of 65% or to produce compound15 with a yield of 88%.

(xv) To compound 13 is added 20 equivalents of Et₃ SiCl in pyridine at25° C. for 20 hours to produce compound 12 with a yield of 85%.

(xvi) To compound 12 is added K₂ CO₃ cat. in MeOH/H₂ O/THF at 0° C. for9 hours with a yield of 85% based on 55% conversion.

(xvii) To the product of (xvi) is added 10 equivalents of phosgene inpyridine at 25° C. for 30 minutes to produce 14 with a yield of 95%.

(xviii) To compound 14 is added 5 equivalents of PhLi in THF at -78° C.for 10 minutes to produce 12 with a yield of 80%.

Definitions: TES=SiEt₂ ; Bz=COC₆ H₅ ; Ac=COCH₃ ; EE=ethoxyethyl.

PHYSICAL CONDITIONS

All new compounds exhibited satisfactory spectral and analytical and/orexact mass data yields refer to chromatographically andspectroscopically homogeneous materials. Selected physical data ispresented as follows:

4: Rf=0.31 (silica, 25% EtOAc in light petroleum); IR (film)=ν_(max)/cm⁻¹ 2926, 1822, 1754, 1732, 1689; ¹ H NMR (500 MHz, CDCl₃); δ 6.52 (s,1 H, 10-H), 4.89 (d, J 9 Hz, 1 H, 5-H), 4.60 (d, J 9 Hz, 1 H, 20a-H),4.48 (d, J 5.5 Hz, 1 H, 2-H), 4.45 (d, J 9 Hz, 1 H, 20b-H), 4.42 (m, 1H, 7-H), 3.49 (d, J 5.5 Hz, 1 H, 3-H), 2.90 (d, J 20 Hz, 1 H, 14a-H),2.79 (d, J 20 Hz, 1 H, 14b-H), 2.56 (m, 1 H, 6a-H), 2.19 (s, 3 H, OAc),2.16 (s, 3 H, oAc), 2.07 (s, 3 H, 18-CH₃), 1.87 (m, 1 H, 6b-H), 1.71 (s,3 H, 19-CH₃), 1.28 (s, 3 H, 16-CH₃), 1.26 (s, 3 H, 17-CH₃), 0.89 (t, J 8Hz, 9 H, SiEt₃), 0.55 (m, 6 H, SiEt₃); ¹³ C NMR (125 MHz, CDCl₃) 200.2,195.7, 170.5, 168.7, 152.0, 150.4, 142.5, 88.2, 83.9, 79.8, 76.6, 75.7,71.5, 61.0, 43.1,41.6, 39.8, 37.7, 3i.6, 29.7, 21.5, 20.7, 18.4, 14.4,9.7, 6.7, 5.1; HRMS (FAB) Calcd. for Ca₃₁ H₄₄ O₁₁ Si (M+H⁺): 621.2731;found 621.2745.

6: Rf=0.5 (silica, 50% EtOAc in light petroleum); IR (film)=ν_(max)/cm⁻¹ 3499, 2956, 1758, 1732, 1673, 1657, 1604; ¹ H NMR (500 MHz, CDCl₃)δ 8.05 (d, J 7.3 Hz, 2 H, OBz), 7.61 (t, J 7.5 Hz, 1 H, OBz), 7.47 (t, J7.8 Hz, 2 H, OBz), 6.57 (s, 1 H, 10-H), 5.67 (d, J 6.7 Hz, 1 H, 2-H),4.90 (d, J 8.4 Hz, 1 H, 5-H), 4.46 (dd, J 10.4, 6.8 Hz, 1 H, 7-H), 4.31(d, J 8.5 Hz, 1 H, 20a-H), 4.09 (d, J 8.5 Hz, 1 H, 20b-H), 3.89 (d, J6.7 Hz, 1 H, 3-H), 2.92 (d, J 19.9 Hz, 1 H, 14a-H), 2.63 (d, J 19.9 Hz,1 H, 14b-H), 2.50 (m, 1 H, 6a-H), 2.21 (s, 3 H, OAc), 2.17 (s, 3 H,OAc), 2.16 (s, 3 H, 18-CH₃), 1.82 (m, 1 H, 6b-H), 1.65 (s, 3 H, 19-CH₃),1.25 (s, 3 H, 16-H), 1.17 (s, 3 H, 17-H), 0.90 (t, J 7.9 Hz, 9 H,SiEt₃), 0.58 (m, 6 H, SiEt₃); ¹³ C NMR (125 MHz, CDCl₃); δ 200.2, 198.3,170.1, 168.9, 166.8, 153.0, 140.2, 133.9, 130.0, 128.8, 128.7.83.9,80.5, 78.4, 76.1, 76.0, 72.8, 72.2, 59.4, 46.2, 43.4, 42.4, 37.1, 33.0,21.7, 21.0, 18.2, 13.5, 9.5, 6.7, 5.1; HRMS (FAB): Calcd for C₃₇ H₅₀ O₁₁Si (M+H.sup. +): 699.3201; found 699.3220.

13: Rf=0.35 (silica, 50% EtOAc in light petroleum); IR (film)=ν_(max)/cm⁻¹ 3503, 2924, 2853, 1728, 1713; ¹ H NMR (500 MHz, CDCl₃); δ 8.06 (d,J 7.3 Hz, 2 H, OBz), 7.58 (t, J 7.5 Hz, 1 H, OBz), 7.45 (t, J 10 Hz, 2H, OBz), 6.31 (s, 1 H, 10-H), 5.58 (d, J 6.5 Hz, 1 H, 2-H), 4.98 (d, J7.5 Hz, 1 H, 5-H), 4.44 (dd, J 11.0, 7.0 Hz, 1 H, 7-H), 4.30 (d, J 8.0Hz, 1 H, 20a-H), 4.14 (d, J 8.0 Hz, 1 H, 20b-H), 3.76 (d, J 6.5 Hz, 1 H,3-H), 2.71 (m, 1 H, 13a-H), 2.55 (m, 1 H, 13b-H), 2.29 (s, 3 H, OAc),2.25 (m, 1 H), 2.23 (s, 3 H, OAc), 1.95 (s, 3 H, 18-CH₃), 1.92 (m, 1 H),1.85 (m, 1 H), 1.69 (m, 1 H), 1.64 (s, 3 H, 19-CH₃), 1.11 (s, 3 H,16-H), 1.09 (s, 3 H, 17-H); ¹³ C NMR (125 MHz, CDCl₃); 204.4, 171.5,169.8, 166.9, 144.0, 133.7, 131.3, 130.0, 129.3, 128.6, 84.22, 81.32,80.97, 76.35, 73.95, 72.39 65.86, 58.89, 45.89, 42.14, 35.71, 30.19,29.69, 26.58, 25.30, 22.08, 20.96, 19.61, 15.27, 9.08; HRMS (FAB): Calcdfor Ca₃₁ H₃₈ O₁₀ (M+Na⁺): 593.2363; found 593.2360.

14: Rf=0.82 (silica, 50% EtOAc in light petroleum); IR (film)=ν_(max)/cm⁻¹ 2924, 1814, 1728, 1461, 1372, 1238; ¹ H NMR (500 MHz, CDCl₃); δ6.40 (s, 1 H, 10-H), 4.95 (d, J 9.0 Hz, 1 H, 5-H), 4.60 (d, J 9.0 Hz, 1H, 20a-H), 4.47 (d, J 9.0 Hz, 1 H, 20b-H), 4.43 (dd, J 10.0, 7.5 Hz, 1H, 7-H), 4.39 (d, J 5.5 Hz, 1 H, 2-H), 3.36 (d, J 5.5 Hz, 1 H, 3-H),2.71 (m, 1 H, 13a-H), 2.56 (m, 1 H, 13b-H), 2.17 (s, 3 H, OAc), 2.15 (s,3 H, OAc), 2.12 (m, 1 H), 2.07 (s, 3 H, 18-CH₃), 1.97 (m, 1 H), 1.88 (m,2 H), 1.78 (s, 3 H, 19-CH₃), 1.23 (s, 3 H, 16-CH₃), 1.17 (s, 3 H,17-CH₃), 0.88 (t, J 7.5 Hz, 9 H, OSiEt₃), 0.55 (dq, J 8.0, 3.0 Hz, 6 H,--OSiEt₃); ¹³ C NMR (125 MHz, CDCl₃); 202.6, 170.3, 169.2, 153.1, 144.0,130.7, 92.8, 84.0, 80.3, 80.0, 76.4, 76.1,60.3, 43.5, 38.0, 29.7, 29.4,25.5, 23.1, 21.9, 21.1, 19.1,9.8, 6.7, 5.2; HRMS (FAB) Calcd. for C₃₁H₄₆ O₁₀ Si (M+Cs⁺): 739.1915; found 739.1929.

What is claimed is:
 1. A taxoid intermediate represented by thefollowing structure: ##STR5##
 2. A taxoid intermediate represented bythe following structure: ##STR6##
 3. A taxoid intermediate representedby the following structure: ##STR7##